1. Field of the Invention
The present invention relates to scanning apparatus for scanning a cassette of the type used in photo-stimulable luminescence ("PSL") radiography and comprising a PSL plate and a cap therefor.
2. Description of the Prior Art
In conventional radiography ("X-ray photography"), a film plate is made by forming one or more silver halide emulsion layers on a flexible film base which is supported within a light-tight cassette. The interior of the cassette is coated with one or more X-ray sensitive luminescent layers. The cassette containing an unexposed X-ray film plate is loaded into an X-ray machine, and after exposure the cassette and exposed X-ray film plate are removed for development and fixing of the latent image produced. This is usually done automatically by feeding the cassette into a light-tight apparatus in which the cassette is opened, and the exposed film plate is extracted and passed through a series of troughs containing the various chemical processing solutions required. The processed plate may also be dried in the apparatus. Meanwhile, a new, unexposed film plate has been loaded into the cassette which is then re-closed, and the reloaded cassette and developed film plate are delivered to respective exit slots of the processing apparatus.
In the PSL system, a PSL X-ray plate has applied thereto a layer of a photostimulable luminescent material which comprises a stimulable phosphor, for example a europium-activated barium fluorohalide, and a binder. The phosphor has the characteristic that it can be energised to an excited state by X-rays, and can then be stimulated by visible or infra-red light to return to the ground state with the emission of visible light (of a different wavelength from the stimulating light). The excited state has a half-life of at least several hours or days in the absence of stimulating light. A PSL plate is potentially re-usable many times. The technique is described in an article by Sonoda et al. in Radiology, Volume 148 (September 1983), at pages 833 to 838, and it offers the potential advantages of better image resolution at lower X-ray dosages for the patient.
The stimulable phosphor is deposited as a layer on a flexible base which also requires enclosure in a light-tight cassette.
Current practice in PSL radiography is to pass the exposed PSL plate in its cassette to an automatic processing machine in which the PSL plate is removed from the cassette, scanned, exposed overall to light to return the PSL material to its ground state and then reloaded into the same cassette for reuse. For scanning, the exposed PSL plate is transported past a laser, typically a helium-neon laser emitting at a wavelength of 633 nm, which scans line-wise across the plate in front of a light-guide comprising a bundle of optical fibres whose input ends are arranged in a line across the path of the plate close to the laser scanning line for the reception of light emitted, typically at wavelengths close to 400 nm, when the PSL material is stimulated by the laser. The light-guide is arranged to pass the emitted light to a photo-multiplier tube or other receptor. The result is a storable electronic raster image. The electronic image may be subjected to any desired computer image-enhancement techniques and it may be displayed on a video display unit, fed to a laser printer for the production of a plain paper copy, or used to control a laser arranged to expose correspondingly a photographic film plate to produce an X-ray plate of conventional appearance.
It will be appreciated that the organisation and operation of a PSL system are analogous to those of a photographic emulsion system. In a photographic emulsion system, the exposed film plate is transferred to a processing apparatus in which the latent image on the film plate is chemically developed and the developed image is scanned. In a PSL system, the film plate of the photographic emulsion system is replaced by the PSL plate. This plate is exposed in the X-ray machine, and is then transferred to a scanning apparatus in which the latent image is scanned by a scanning beam which "develops" the image in the process of scanning.
The components of PSL systems have therefore hitherto been designed to match those of photographic emulsion systems as far as feasible, to take advantage of the design and development investment and experience which has been accumulated with photographic emulsion systems. In particular, the plates of PSL material used in PSL systems and their handling have generally been designed to match those of photographic emulsion systems. In such PSL systems, the PSL material is thus in the form of a coating on a flexible film which is removable from a cassette, and little attention has been paid to the design of PSL scanning apparatus other than to replace the chemical processing stage with a physico-optical stage and to arrange for the cassette to be reloaded with the same PSL plate after erasure, as opposed to reloading with a new, unexposed photographic plate.
Cassettes in a known PSL system are conventional X-ray cassettes, comprising a base and a lid hinged thereto, both parts having at the inside a soft lining for protection of the enclosed PSL sheet. Systems using suchlike cassettes are voluminous because opening the lid of a cassette must occur over a certain minimum angle to get sufficient access to the interior for a suction cup to catch the film and remove it from the cassette. In addition, manipulation of the locks of the cassette, of the hinging cap, and delicate gripping and transport of the exposed PSL sheet require a considerable amount of precise, mechanical gripping means and co-operating servo-motors making control of the system complicate and requiring additional space. A system of the kind described is disclosed in EP-A-0 309 874.
An example of a photographic cassette comprising a separate cap is disclosed in EP-A 0 347 647. This cassette has the disadvantage that the support for the screen, i.e. the cap, is flexible whereby its manipulation becomes more delicate.